As semiconductor layers used for display devices, much attention has been paid to oxide semiconductors. Compared with widely used amorphous silicon (a-Si), oxide semiconductors have high carrier mobility, high optical band gaps, and easy film formability at low temperatures, and therefore, have highly been expected to be applied for next generation displays, which are required to have large sizes, high resolution, and high-speed drives; low heat-resistant resin substrates; and other uses.
Oxide semiconductors each contain at least one element selected from the group consisting of In, Ga, Zn, and Sn, and typical examples of the oxide semiconductors may include, for example, In-containing oxide semiconductors (In—Ga—Zn—O, In—Zn—Sn—O, and In—Zn—O etc,). There have also been proposed Zn-containing oxide semiconductors (Zn—Sn—O and Ga—Zn—Sn—O etc,) as the oxide semiconductor containing no In as a rare metal, thereby making a reduction in material cost and being suitable for mass production (e.g., Patent Document 1).